1. Field of the Invention
The present invention relates to a resin composition for sealing electronic parts, which is excellent in thermal conductivity and metal abrasion resistance. Furthermore, the present invention relates to a magnesia powder in which the hydration resistance is highly improved by covering the surfaces of particles of the magnesia powder with a uniform coating of silica, and a process for the preparation thereof.
2. Description of the Related Art
The method for resin-sealing electronic parts such as semiconductors, resistors, condensers and coils with thermosetting molding materials is vigorously adopted in these years. The requirement for the reliability of each electronic part is increasing and hence, improvements of characteristics of sealants are eagerly desired. For example, the degree of integration of semiconductors is increased and the quantity of heat generated is increased with increase of the integration degree, and therefore, the heat dissipation of resin-sealed electronic parts becomes a problem. For solving this problem, use of a sealant having a high thermal conductivity is desired.
From the economical viewpoint and in view of heat resistance, moisture resistance and adhesion, in most of conventional sealants, an epoxy resin is used as the thermosetting resin and silica powder is added as an inorganic filler for improving thermal conductivity or mechanical strength. Most of available silica powders are composed of crystalline silica or fused silica. Resin-sealing materials comprising these silica powders, however, are still defective in that the thermal conductivity is not sufficient to seal a highly integrated electronic part having a large quantity of generated heat. Furthermore, since the hardness of silica powder is high, a resin-sealing material comprising silica powder wears a molding machine or mold at the transfer molding or injection molding, and this sealing material is not suitable for the long-period operation. Therefore, development of an improved a resin-sealing material is desired.
Magnesia is excellent in heat resistance, thermal conductivity and electrically insulating property and a high-density sintered body of magnesia has a high transparency. By dint of these characteristics, utilization of magnesia powder as the material of fillers or magnesia ceramics has been examined. However, magnesia powder is practically used only as a material of heat-resistant magnesia ceramics such as a crucible or a protecting tube of a thermocouple. The reason is that magnesia is poor in the hydration resistance and especially in the powdery state, magnesia is readily hydrated by water contained in air and converted to magnesium hydroxide, resulting in drastic degradation of characteristics. As means for coping with this hydration, there has been adopted a coupling treatment using such a coupling agent as 3-aminopropyltriethoxysilane, phenyltrimethoxysilane or 3-methacryloxypropyltrimethoxysilane.
However, even if this coupling treatment is carried out, magnesia powder having a sufficiently high hydration resistance cannot be obtained, and magnesia powder which has been subjected to the coupling treatment cannot be used as a filler or the like. Moreover, the coupling treatment requires complicated steps of filtration, drying, rough pulverization and calcination, and the treatment is very expensive.